The present invention is directed to an activated starter mixture which can be used to prepare polyoxyalkylene polyols. The present invention is also directed to a process for preparing an activated starter mixture, particularly, to a process for preparing an activated starter mixture which is composed of a low molecular weight starter compound. The present invention is also directed to a batch or semi-batch process for the polyaddition of an alkylene oxide on to an activated starter mixture, particularly, on to an activated starter mixture which is composed of a low molecular weight starter compound.
Base-catalyzed oxyalkylation processes have been used to prepare polyoxyalkylene polyols for many years. In base-catalyzed oxyalkylation processes, suitable low molecular weight starter compounds, such as propylene glycol or glycerine, are oxyalkylated with alkylene oxides, for example, ethylene oxide or propylene oxide, to form polyoxyalkylene polyols. Reactor capacity is effectively utilized in base-catalyzed oxyalkylation processes due to the fact that the build ratio (polyol weight/starter weight) is relatively high as a result of using low molecular weight starter compounds in the process.
However, to a varying degree, basic catalysts catalyze an isomerization of propylene oxide to form allyl alcohol. Allyl alcohol acts as a monofunctional initiator during the polymerization of propylene oxide. Thus, when a basic catalyst, such as potassium hydroxide, is used to catalyze a propylene oxide polymerization, the product contains allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the product being polymerized increases, the isomerization reaction becomes more prevalent. As a result, 800 or higher equivalent weight poly(propylene oxide) products prepared using KOH as the catalyst tend to have significant quantities of monofunctional impurities. This tends to reduce the average functionality and broaden the molecular weight distribution of the product. Generally, polyols having higher average functionalities typically produce polyurethane products with better physical properties.
Double-metal cyanide (xe2x80x9cDMCxe2x80x9d) catalysts can be used to produce polyols which have low unsaturation levels and more narrow molecular weight distributions compared with polyols produced using KOH catalysis. DMC catalysts can be used to produce polyether, polyester and polyetherester polyols which are useful in polyurethane coatings, elastomers, sealants, foams and adhesives.
DMC catalysts are typically obtained by reacting an aqueous solution of a metal salt (for example, zinc chloride) with an aqueous solution of a metal cyanide salt (for example, potassium hexacyanocobaltate), in the presence of an organic complexing ligand. The preparation of a typical DMC catalyst is described in, for example, U.S. Pat. Nos. 3,427,256, 3,289,505 and 5,158,922.
Organic complexing ligands are needed in the preparation of DMC catalysts in order to obtain favorable catalytic activity. While water-soluble ethers (e.g., dimethoxyethane (xe2x80x9cglymexe2x80x9d) or diglyme) and alcohols (for example, isopropyl alcohol or tert-butyl alcohol) are commonly used as the organic complexing ligand, other general classes of compounds have been described which are useful as the organic complexing ligand. For example, U.S. Pat. Nos. 4,477,589, 3,829,505 and 3,278,459 disclose DMC catalysts containing organic complexing ligands selected from alcohols, aldehydes, ketones, ethers, esters, amides, nitriles and sulphides.
DMC catalysts having increased activity for epoxide polymerization are known. For example, U.S. Pat. Nos. 5,482,908 and 5,545,601 disclose DMC catalysts having increased activity which are composed of a functionalized polymer such as polyether.
In the presence of DMC catalysts, however, conventional low molecular weight starter compounds (such as water, propylene glycol, glycerine and trimethylolpropane) initiate oxyalkylation sluggishly (if at all), particularly in a typical batch process for the preparation of polyols. Long catalyst initiation times increase reaction cycle time and can lead to deactivation of the DMC catalyst. As a result, in a typical batch or semi-batch process, starter compounds having a high molecular weight are typically used.
High molecular weight starter compounds which are used in DMC-catalyzed alkoxylation processes are typically prepared by alkoxylating low molecular weight starter compounds, such as glycerine, in the presence of a basic catalyst, such as KOH, to produce alkoxylated polyol starters of several hundred molecular weight. Such starter compounds are refined to remove KOH residues and then alkoxylated in the presence of DMC catalysts to produce polyether polyols of several thousand molecular weight. The base catalyst must be removed from the starter compound before the starter compound can be used as an initiator in a DMC-catalyzed oxyalkylation process because even traces of basic substances often de-activate DMC catalysts.
A process for preparing polyether polyols using DMC catalysis which eliminates the need to synthesize costly high molecular weight starter compounds by KOH catalysis in a separate, dedicated reactor is described in, for example, U.S. Pat. No. 6,359,101. However, the process described in this patent is limited to activating specific low molecular weight starter compounds in the presence of DMC catalysts under specific reaction conditions.
Another process for preparing polyether polyols using DMC catalysis which eliminates the need to synthesize costly high molecular weight starter compounds by KOH catalysis is described in, for example, U.S. Pat. No. 5,767,323. This patent describes using pre-initiated initiator/alkylene oxide/catalyst master batches which have decreased induction periods. This patent discloses adding one or more initiators having an equivalent weight of from 100 Da to 500 Da and catalyst to a reactor and, after N2 flushing, adding an initial quantity of alkylene oxide until a pressure drop occurs. Preferably, alkylene oxide is added to the activated starter mixture but, optionally, the activated starter mixture can be mixed further with additional starter compound, specifically, a starter compound of the same or of a high molecular weight. Oxyalkylation can then be commenced without an appreciable induction period.
In typical batch or semi-batch processes for producing DMC-catalyzed polyols, high molecular weight starter compounds and DMC catalysts are charged to a reactor all at once. One drawback of charging starter compounds to a reactor all at one time is inefficient use of reactor capacity. For example, the preparation of a 3000 Da molecular weight polyoxypropylated glycerine triol may be achieved through oxypropylation of a 1500 Da molecular weight oligomeric oxypropylated glycerine starter until a molecular weight of 3000 Da is achieved. The build ratio is 3000 Da/1500 Da or 2.0. This low build ratio cannot efficiently take advantage of reactor capacity, as some 40% of the total reactor capacity is used just for the starter compound.
U.S. Pat. No. 5,689,012 describes a process for producing DMC-catalyzed polyols which makes effective use of reactor capacity while at the same time effectively using low molecular weight starter compounds. The process described in this patent, however, is directed to continuously adding low molecular weight starter compounds to a reactor rather than charging high molecular weight starter compounds to a reactor all at one time (such as in a batch or semi-batch process).
U.S. Pat. No. 5,777,177 also describes a process for producing DMC-catalyzed polyols which makes effective use of reactor capacity while at the same time effectively using low molecular weight starter compounds. The process disclosed in U.S. Pat. No. 5,777,177 describes producing DMC-catalyzed polyols by continuously feeding propylene oxide and low molecular weight starter compounds (such as, for example, water, propylene glycol, glycerine or trimethylol propane) to a reactor along with an additional feed of propylene oxide and catalyst after polymerization has been initiated with a high molecular weight starter compound.
The process described in U.S. Pat. No. 5,777,177, however, requires that a low concentration of a low molecular weight starter compound always be maintained in the reactor so that the low molecular weight starter compound is consumed at the same rate at which it is added to the reactor. There therefore remains a need for an improved batch or semi-batch process for the polyaddition of an alkylene oxide on to a starter compound, particularly, on to a low molecular weight starter compound.
The present invention is directed to an activated starter mixture which can be used to prepare polyoxyalkylene polyols. The present invention is also directed to a process for preparing an activated starter mixture, particularly, to a process for preparing an activated starter mixture which is composed of a low molecular weight starter compound. The present invention is also directed to batch or semi-batch process for the polyaddition of an alkylene oxide on to an activated starter mixture, particularly, on to an activated starter mixture which is composed of a low molecular weight starter compound.